In the field of lithographic printing plates, so-called presensitized printing plates comprising an aluminum support having thereon a light-sensitive layer composed of a light-sensitive composition have hitherto been employed. The aluminum support used in the presensitized lithographic printing plates generally has a surface roughened by a process selected from various roughening processes, such as mechanical roughening processes including ball graining, wire graining, brush graining, and liquid honing; an electrochemical roughening process called electrolytic graining; chemical roughening processes; and certain combinations of two or more of these processes.
Typically employed roughening processes include a mechanical roughening process, an electrochemical roughening process, and certain combinations thereof, but each of these techniques has respective disadvantages as set forth below.
Wire graining is unsuitable for obtaining commercially usable prints of high quality due to the simple structure of the wire. The brush graining involves orientation of the rotating brush, which results in a non-uniform surface of the aluminum plate. Electrolytic graining requires a large energy for attaining a desired roughness and precise control of electrolysis conditions for stably obtaining a constant surface quality.
In order to attempt to overcome some of the above-described disadvantages associated with each graining process, an improved process comprising a combination of the brush graining or wire graining and the electrolytic graining has been proposed, as disclosed in U.S. Pat. No. 2,344,510 and Japanese patent application (OPI) No. 123204/78 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), and British Pat. Nos. 1,582,620 and 2,047,274. According to such combined process, however, when the brush graining is adopted as a first step, i.e., a mechanical graining step, stains are apt to be formed in the non-image areas during printing, and when the wire graining is adopted as the first step, the printing plate has poor printing durability.